This invention relates to pressure-sensitive devices. More particularly, the invention concerns structures for accurately localizing point sources of pressure and methods for determining the location and force of a point pressure source on a pressure-sensing surface. The invention may be applied to provide interfaces to electronic devices.
It is known to make pressure-sensitive surfaces by instrumenting a mat or other structure which includes a surface region with an array of pressure sensing elements. For example, an array of pressure sensors of the type described in PCT publication No. WO 99/04234 (Reimer, et al.), can be used to detect the location and pressure applied by several simultaneous points of contact.
The surface may be covered with a membrane as described, for example, in WO 00/73982 (Inkster). Such membranes can cause problems, however. A membrane can distribute pressure so that touching at one location causes signals from the pressure sensors in a surrounding area. The prior pressure-sensitive surfaces described above provide no way to isolate the response of a set of pressure sensors from neighboring ones.
In conventional pressure-sensitive structures which include a membrane overlying an array of pressure sensors, the membrane distributes pressure imposed by an indentor radially outwardly from the indentor in a generally uniform manner. The inventors have determined that by selecting the properties of a membrane, it is possible to control how force from an indentor will be distributed over pressure sensors in a pressure-sensitive structure. This can be very beneficial in some applications.
In general, the invention relates to position sensing surfaces wherein individual sensing elements located on a substrate measure pressure at specific locations. In many cases it is desirable to know the location of the applied pressure, which may not be directly over one of the sensing elements. In such applications, a membrane positioned adjacent the substrate can distribute the pressure over several nearby pressure sensing elements, which enables one to compute the pressure at any point by interpolating between sensors. This also provides a means to reduce the total number of sensing elements for a resulting reduction in costs and complexity.
The sensitivity of the device may be improved by the use of protrusions on the membrane, each of which is located so as to contact an individual sensing element. As can be imagined, when a membrane is constructed in this manner, any force applied to the membrane is transferred to the individual sensing elements by the protrusions. Since the total contact area of the protrusions is small, the bearing pressure is concentrated at those protrusions.
If an xe2x80x9cindentorxe2x80x9d is used to apply a force to the surface, the location of the indentor may be estimated by means of a centre-of-mass algorithm or other similar mathematical computation. The invention provides to an improved method to accurately compute the indentor location.
The invention further relates to a position sensor comprising a substrate covered by a membrane. The substrate comprises an array of pressure detecting means. The pressure detecting means may comprise any of a number of systems, including force sensitive resistors, piezo-electric crystals, strain gauge-based sensors, and optical pressure sensors of the type described in WO 99/04234, among others. The type of pressure detecting means may be varied without departing from the invention. One aspect of the invention relates to the use of physical features in the membrane (such as holes or recesses) to tailor the distribution of pressure. Another aspect of this invention relates to position sensors having irregular distributions of pressure sensing elements. Another aspect of this invention provides a way to accurately compute the location of an xe2x80x9cindentorxe2x80x9d which is applying pressure.
In one aspect, the invention comprises a position sensor, comprising a substrate covered by a membrane. The substrate includes an array of individual pressure-detecting elements. The membrane has a non-uniform structure and comprises means for isolating areas of depression of the membrane caused by local application of pressure wherein depression of a first of said areas causes substantially no depression of a second, adjacent area.
The means for isolating the membrane areas may comprise a slot between the first and second areas. Alternatively, the isolating means may comprise an area in which the membrane is fixed to the substrate. The membrane may be fixed to the substrate in a trough, or depressed portion between the areas. The portions of the membrane adjacent to the trough are not fixedly engaged to the substrate.
In one embodiment, the membrane comprises at least one depressed region for contacting the substrate. The membrane is separated from the substrate other than at the location at the at least one depressed region.
The isolated membrane areas have various shapes and may be arranged in regular or irregular arrays. The areas may be one or more of a rectangular, triangular, truncated triangular or irregular-shaped.
In another aspect, the membrane is partly separated from the substrate, being supported on the substrate by one or more regions of the membrane which contact the substrate. Pressure applied to the membrane is thus transmitted to the substrate solely or substantially at the contact regions.
In another aspect, the invention comprises a pressure sensing surface comprising a substrate having an array of pressure sensing means and signal processing means to receive pressure information from said sensing means. The signal processor means is programmed to calculate the location and magnitude of force applied to the membrane according to a formulae described herein, in which the sensor is assumed to be generally planar with x, y coordinates describing its surface.
This specification includes directional references such as xe2x80x9cupxe2x80x9d and xe2x80x9cdownxe2x80x9d for convenience and ease of understanding. It will be understood that the sensors described herein may be placed in any orientation. The directional references herein are not intended to be limit the invention.